Weather Service Report

FXUS66 KPQR 240350

Area Forecast Discussion
National Weather Service Portland OR
843 PM PDT Mon May 23 2016

.SYNOPSIS...A broad area of low pres over the region will gradually 
drop further southward, resulting in drier weather for Tuesday and
Wednesday with near normal temperatures. This break will be short
lived as another upper low pressure will arrive to close the week.
This will bring showers and cooler temperatures to the region for
the coming holiday weekend. 


.SHORT TERM...(Tonight through Thursday)...A broad upper level trough
apparent on water vapor imagery will more or less continue to linger
across the Pacific Northwest and western US for the next several
days. With that said, the atmosphere will gradually warm and dry out
aloft Tuesday and Wednesday as weak shortwave ridging tries to
temporarily nose into the region from the west. This should bring
considerably fewer showers to the area when compared to the past
several days.

In the immediate short term...visible satellite imagery and doppler
radar indicate plenty of showers developed this afternoon and early
this evening. Showers have generally been fairly weak with limited
instability and equilibrium levels generally remaining under 15kft.
An area of weaker flow, in part due to a weak low pressure spinning
southward offshore, did allow a few slow moving showers to
develop over the north Oregon coast range, but even here rainfall
amounts have generally remained under a quarter of an inch. With the
loss of daytime heating, expect showers to decrease overnight with
minimal rain chances by morning.

As 500mb heights build overhead tonight and Tuesday the lower to
middle part of the atmosphere overhead will warm and dry out when
compared to today. This should produce considerably fewer showers
Tuesday. Nonetheless, there may be just enough instability present
during the afternoon to allow a few showers to develop over the
central Oregon coast range and particularly the Oregon Cascades.
There is a slight chance a shower or two will drift into the central
and southern Willamette Valley during the late afternoon, but most
folks should remain dry. Models are noticeably drier and more stable
farther north towards PDX so PoPs were nudged below mentionable

Models in varying shapes and forms, hint that a weak low level
front/trough will push south-southeastward towards the Pacific
Northwest late Tuesday night and Wednesday morning. This front
appears to be the last gasps of low level feature associated with the
shortwave trough apparent on water vapor imagery currently diving
south-southeastward across the Alaska Panhandle. Model soundings and
QPF fields indicate enough low level moisture and instability will
move into the region that a few showers seem plausible along the
coast and coast range early Wednesday. Given the shallow nature of
this moisture instrusion, generally below 850mb, not sure how much
the chance for wet weather will extend inland. Nonetheless, a few
light showers may develop across the interior in the late
morning/early afternoon as the lower atmosphere turns over,
particularly across the north. PoPs were trended towards this general
evolution in mind.  

Models are in modest agreement that a shortwave trough currently over
western Alaska will slide south-southeastward and approach the region
Thursday. This should reinforce marine clouds and keep temperatures
cool. It may increase light shower chances, but model soundings are
not terribly impressive so at this point, it appears any showers that
develop would be very light. And there is a decent chance showers
associated with this system may very well hold off until Thursday
night...or Friday for that matter. /Neuman

.LONG TERM...Thursday night through Monday. Previous discussion
below...Forecast models are in reasonable agreement for the first
part of the weekend in terms of the overall pattern. An upper low
dropping into southern B.C. late Thursday will slide south across
Washington and Oregon on Friday. This will support another round of
widespread showers across the region and afternoon temperatures will
once again be a few degrees below seasonal norms. Some slight
differences in how the models evolve this upper low, but the general
trend in the latest GFS and ECMWF reflect that the low will push
south and east and allow heights to begin to rise over the region by
the end of the weekend. That said, ensemble remembers still have a
significant spread of possibilities and the GEM would also keep some
showers lingering through the weekend. All of that said, did maintain
some slight chance PoPs through the weekend, but tapered these off
later Sunday into Monday. If this ridge does begin to push over the
Pacific NW by the end of the weekend, expect slightly warmer
temperatures and decreasing clouds -though this still remains a lower
confidence forecast at this time for the reasons stated above. Snow
levels for the weekend may be as low as 5,000 feet late Friday, but
increasing to generally above 6,000 feet for the rest of the weekend.


.AVIATION...Scattered showers this evening will taper off
considerably overnight. VFR conditions currently prevailing across
the region, with cigs around 5 to 8 kft. VFR should persist
through most of the night. Cigs likely will settle a bit
overnight over the interior. There is a chance that cigs may
briefly drop to 2500-3000 ft between 12Z and 18Z Tue, but think
it is more likely that VFR conditions persist over the interior.
The fcst models indicate that MVFR or IFR stratus will reform
along the coast later tonight...likely 09Z-12Z as northerly flow
becomes established. Any lower clouds that manage to form Tue AM
should lift to VFR by midday, with widespread VFR expected Tue
afternoon and evening. The mountains will remain mostly obscured
into Tue AM.

PDX AND APPROACHES...Expect mainly VFR conditions to prevail
through Tue evening. There is a chance that cigs will lower to
around 2500-3000 ft between 12Z and 18Z Tue, but think it is more
likely that conditions remain VFR. Pyle


.MARINE...High pressure building over the waters this evening,
turning winds northerly overnight into Tuesday morning. High
pressure remains over the waters through much of the week with
northerly winds. Will see winds increase to near 20 knots each
afternoon/evening starting Tuesday through the end of the week,
with weaker winds expected each morning. As of now, not seeing
winds get much above 20 knots. Gradients don't appear to be strong
enough until possibly Sunday as an upper-level low drops south out
of British Columbia.

Seas are currently around 6 to 7 ft, with dominant wave periods
around 9 seconds. With persistent northerly winds, we will see
fresh swell start to build into the waters from the north-
northwest, which could bring hazardously steep seas to the outer
waters beyond 10 nm possibly as soon as Tuesday evening, but more
likely Wednesday afternoon and evening. Seas will build to around
8 ft with dominant wave periods dropping to around 8 seconds by
Wednesday evening. Outer waters beyond 10 nm from the Coast will
probably see steep seas around 7 to 8 ft with wave periods around
8 seconds through Friday, though there is a chance that seas will
be less steep around 6 ft during morning hours when winds will be
weaker. Over the weekend, the upper-level low may disrupt the
northerly wind pattern to our north, which could allow seas to
become less steep. Right now, though, much uncertainty exists with
the track of this system. Pyle/LM





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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.

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